Authorities tell residents of three towns north of Boston to remain vigilant for any signs of gas or smoke

BOSTON—The series of deadly gas explosions and fires in three communities north of Boston last week occurred after too much natural gas was pumped into a section of pipe owned by Columbia Gas, causing the combustible fuel to leak into homes, authorities said Sunday.

The National Transportation Safety Board “can confirm at this time that this was indeed an overpressure situation,” NTSB Chairman Robert Sumwalt said at a news briefing.